The myotonic dystrophies [DM], including myotonic dystrophy type 1 [DM 1] and myotonic dystrophy type 2 [DM2], are dominantly inherited, multisystem diseases that share the major disease manifestations of myotonia, wasting ad weakness. DM1 is caused by an unstable CTG repeat expansion on chromosome 19q13.3 and DM2 by an unstable CCTG repeat expansion 3q21. The cause for the skeletal muscle findings in these disorders remains a mystery. We hypothesize that they result from toxic effect of mutant RNA, a RNA mediated gain of function. Support for our hypothesis comes from studies in a mouse model of DM1. The muscles of the mice show a focal accumulation of expanded CUG repeats in the nucleus. These nuclear foci closely resemble the RNA foci of expanded CUG repeats in DM1 and expanded CCUG repeats in DM2. Expression of the expanded CUG repeat in the transgenic model causes myotonia, like that seen in DM1 and DM2. We have obtained evidence that myotonia in the transgenic model results from aberrant splicing of the chloride channel 1(CIC-1) pre-mRNA, and consequent loss of CIC-1 protein and chloride conductance from the muscle membrane. We hypothesize that a similar RNA-mediated disturbance of RNA processing underlines myotonia and other disease manifestations in DM1. To test this hypothesis we will evaluate patients with DM1 and DM2, disease controls, and normals using clinical and electromyographic testing, needle muscle biopsy, tissue culture techniques, studies of RNA expression and splicing, immunofluorescence microscopy, and microarray analysis to address four specific aims. These aims are: 1) define the mechanism of myotonia - analyze splicing of CIC1 inDM1 and DM2; 2) determine the extent of mis-splicing (beyond CIC1) in DM1 and DM2; 3) examine the structure/distribution/protein binding characteristics of the nuclear foci containing the expanded repeats in DM1 and DM2; and, 4) assess the expression profile of mRNA inDM1 and DM2. The results of these investigations will shed light on the possibility that RNA gain-of-function represents a fundamental, shared disease mechanism in all forms of myotonic dystrophy. The findings may also help to explain the common clinical manifestations that occur in DM1, DM2, and other myotonic dystrophy like disorders. Ultimately our results may improve our understanding of the pathophysiology of DM and guide us toward the development of new approaches to treatment.